Bad Signs After ACL Surgery: Risks, Complications & When To Worry
Medically Reviewed by
Dr. Hesham Al-Khateeb
FRCS (Orthopaedics) · UK Board-Certified
ACL Surgery Risks & Complications: An OverviewACL surgery (anterior cruciate ligament reconstruction) is a safe, commonly performed procedure with a high success rate. However, like all surgical procedures, there are known ACL surgery risks and potential complications patients should understand before and after the operation. Most complications are minor and treatable when caught early. The most serious ACL surgery complications include: blood clots (DVT), wound infection, graft failure, nerve damage, and arthrofibrosis (excessive scar tissue). This guide, reviewed by Dubai-based knee surgeon Dr. Hesham Al Khateeb, covers every bad sign after ACL surgery — from the first week through 6+ months post-operation — so you know exactly when to seek help. Quick Navigation: Signs of Infection | Blood Clots & DVT | Graft Failure | Excessive Pain | Stiffness | Long-Term Complications | ACL Surgery Side Effects by Month |
Anterior Cruciate Ligament (ACL) reconstruction is a remarkable and highly successful surgery that helps thousands of athletes and active individuals return to their passions every year. The goal is simple: restore stability to the knee and get you back to living your life. For most, the recovery journey, while challenging, is a predictable path of healing and rehabilitation.
However, it’s crucial for patients to be active participants in their recovery. This means not just doing your physiotherapy exercises, but also being vigilant about your body’s signals. While some pain, swelling, and stiffness are normal parts of the process, certain symptoms can be bad signs after ACL surgery that indicate a more serious issue. Ignoring these ACL surgery complications warning signs can lead to significant setbacks, further damage, or long-term problems.
Understanding the difference between a normal recovery hiccup and a true red flag is vital. This is where expert guidance becomes indispensable. This comprehensive guide is provided by Dr. Hesham Al Khateeb, an award-winning, UK-Board Certified Hip and Knee Surgeon. With a background of over 2,000 joint replacement surgeries and specialized fellowship training across the globe, Dr. Al Khateeb brings a wealth of experience in sports injuries and complex knee procedures. His goal is to empower you with the knowledge to navigate your recovery confidently and know exactly when to worry after ACL surgery. Let’s walk through what’s normal, and more importantly, what isn’t.
Understanding ACL surgery and recovery
Before we dive into the warning signs, it’s helpful to understand the typical recovery timeline. Your body has been through a significant procedure, and healing takes time.
The First Week: The first week after ACL surgery expectations should be centered on rest, ice, elevation, and pain management. Swelling and bruising are at their peak. Pain is expected but should be manageable with the prescribed medication. Your primary goals are to control swelling and begin very gentle motion exercises, like ankle pumps and quad sets, as guided by your surgeon and therapist.
Weeks 2-6: During this phase, you’ll focus on weaning off crutches, improving your range of motion, and activating your quadriceps muscle. The pain timeline after ACL surgery should show a steady decrease, and while swelling will still be present, the swelling timeline after ACL reconstruction should also be trending downwards, especially with consistent icing and elevation.
Months 2-6: This is the strengthening phase. You’ll be working hard in physical therapy to rebuild the muscles that support your knee. You should feel your stability improving week by week.
Any significant deviation from this general path—or the appearance of the specific symptoms below—warrants immediate attention.
Read – How Long Does an ACL Tear Take to Heal Without Surgery?
Normal vs Abnormal Symptoms Table
Normal After ACL Surgery | BAD SIGNS — Contact Your Surgeon |
Mild swelling, decreasing over weeks | Sudden large increase in swelling at any stage |
Pain that gradually improves each week | Pain that gets WORSE after initial improvement |
Slight numbness near small incision sites | Numbness/tingling covering large area of leg or foot |
Low-grade fever in first 48 hours (<38°C) | Fever above 38.5°C lasting more than 2 days |
Bruising travelling downward (gravity) | Spreading redness around knee, hot to touch |
Knee feels stiff in first 2-4 weeks | Cannot straighten knee after first 2 weeks |
Mild calf aching from inactivity | Calf pain + swelling + redness = possible DVT |
Clear/light pink wound discharge in first 2 days | Pus, thick white/yellow/green discharge, bad smell |
General leg weakness improving with physio | Knee ‘gives way’ or buckles during walking |
Hardware feels present but not painful | Screw area becomes increasingly painful or prominent |
Critical Red Flags: 5 Categories of Bad Signs After ACL Surgery You Must Not Ignore
Think of these as the major warning categories. If you experience a symptom listed here, it’s not a time to “wait and see.” It’s time to call your surgeon’s office.
1. Signs of Infection: The Top Priority
A surgical site infection is one of the most serious ACL surgery complications warning signs. An infection can compromise the new graft and the entire knee joint, potentially requiring further surgery. It’s crucial to distinguish between normal post-operative healing and the signs of a brewing infection.
What to Look For:
- Increasing Redness, Warmth, and Tenderness: While some pinkness around the incision is normal, watch for redness warmth drainage after ACL surgery that is expanding, becoming a deep, angry red, or feels significantly hot to the touch compared to the other leg.
- Pus or Foul-Smelling Drainage: Clear or slightly yellowish/pinkish fluid might ooze for a day or two. However, thick, cloudy, white, yellow, or green pus, especially with a bad smell, is a major red flag. If you see incision drainage bad smell after ACL surgery, it’s a clear sign of infection.
- Persistent Fever: A low-grade fever (under 100.4°F or 38°C) can be common in the first 48 hours. However, a fever after ACL surgery what to do becomes a critical question if it’s above 101°F (38.5°C) or persists for more than two days. This systemic response indicates your body is fighting a significant battle.
- Severe, Worsening Pain: Post-operative pain should gradually improve. If your knee pain suddenly gets much worse, becomes throbbing, and isn’t relieved by medication or rest, it could be one of the key infection after ACL reconstruction symptoms.
The Danger: An untreated infection can lead to septic arthritis after ACL reconstruction, a condition where bacteria enter the joint space itself. This can destroy cartilage and lead to failure of the graft. A deep joint infection after ACL surgery is a surgical emergency.
Action Plan: If you suspect an infection, contact your surgeon’s office immediately, day or night. Do not wait for your next scheduled appointment.
2. Blood Clots (DVT) After ACL Surgery: Symptoms, Risk & Prevention
DVT after ACL surgery (deep vein thrombosis) is one of the most serious potential complications, occurring in an estimated 1–5% of cases. ACL surgery blood clot formation is triggered by reduced blood flow during immobility after the operation.
Symptoms of Blood Clot After ACL Surgery:
- Calf pain after ACL surgery — cramping, aching, or charley-horse sensation in the lower leg (not at the knee)
- Calf pain after ACL surgery with hamstring graft — particularly common as the graft harvest site can mask or confuse calf symptoms; monitor carefully
- Swelling in the calf that does not improve with elevation
- Skin that is red, warm, or feels hard in one calf area
- Why does my calf hurt after ACL surgery? — If pain is specifically in the back of the lower leg, not the knee itself, DVT must be ruled out before attributing it to muscle soreness
Pulmonary Embolism — Emergency Signs:
CALL 999 / GO TO A&E IMMEDIATELY IF: • Sudden shortness of breath • Chest pain, especially when breathing in • Coughing up blood • Rapid heart rate or lightheadedness This is a pulmonary embolism — a blood clot in the lungs — and is a life-threatening emergency. |
DVT Prevention After ACL Surgery:
- Ankle pumping exercises every 1–2 hours when awake
- Compression stockings as prescribed by your surgeon
- Anticoagulant medication if prescribed
Return to walking as soon as safely possible (reduces clot risk significantly)
3. Graft Failure and Instability: That “Giving Way” Sensation
The entire point of ACL surgery is to restore stability. If your knee continues to feel unstable, it’s a significant concern.
What to Look For:
Episodes of “Giving Way” or Buckling: This is the classic ACL graft failure signs giving way. It’s a distinct feeling of your knee suddenly shifting or being unable to support your weight during simple activities like walking or pivoting
Persistent Feelings of Looseness: As you recover, your knee should feel progressively more solid. If you have persistent feelings of looseness knee after ACL surgery, or it just doesn’t “feel right,” it’s worth discussing with your surgeon. It’s important to ask, “instability after ACL reconstruction is it normal?” While some perceived instability can be due to muscle weakness (quad activation failure after acl surgery is common), true mechanical instability is not normal.
Action Plan: Document any instances of instability—what you were doing, how it felt, and the swelling that followed. Report this to your surgeon at your next visit, or sooner if it happens frequently or results in a fall.
4. Excessive Pain and Swelling: When It’s More Than Just Healing
Pain and swelling are universal after surgery, but they should follow a downward trend. When they don’t, it’s a sign that something is wrong.
What to Look For:
Severe, Uncontrolled Pain: If you have severe pain after ACL surgery not improving with your prescribed medication, rest, and ice, it’s a red flag. Pain that is out of proportion to the stage of your recovery is one of the most important bad signs after ACL surgery.
Dramatic and Sudden Swelling: While some swelling persists for months, a sudden, large increase in fluid—a persistent effusion after ACL reconstruction—can signal a problem like bleeding in the joint (hemarthrosis), an internal injury, or infection. Understanding excessive swelling after ACL surgery what it means is key; it’s your body’s emergency signal.
Bruising That Worsens or Spreads: Bruising will travel down your leg due to gravity. However, if bruising getting worse after ACL surgery in the area of the knee itself, combined with increased pain and swelling, it could indicate bleeding.
Action Plan: If your pain is uncontrollable or your knee suddenly swells up like a balloon, contact your surgeon’s office. This could indicate bleeding into the joint or another internal issue that needs to be addressed.
5. Severe Stiffness and Loss of Motion: The “Frozen Knee” Risk
Gaining back your full range of motion, especially the ability to straighten your knee (extension), is a critical early goal. A failure to do so is a major problem.
What to Look For:
Inability to Straighten the Knee: One of the most significant knee stiffness after ACL surgery red flags is the loss of extension. If you cannot fully extend knee after ACL surgery after the first couple of weeks, it’s a serious issue. A bent-knee gait can lead to long-term cartilage problems and pain.
Progressive Loss of Motion: If you find your range of motion is actually decreasing despite your efforts in physical therapy, you may be developing Arthrofibrosis.
Arthrofibrosis (Scar Tissue): These are the arthrofibrosis after ACL reconstruction signs: a feeling of a “hard block” at the end of your range of motion, not just muscle tightness. The knee feels genuinely stuck. This excessive scar tissue formation can severely limit your function and often requires another procedure to resolve.
Action Plan: This is a problem to tackle early. Constant communication with your physical therapist is key. If you are not progressing in acl physio warning signs like this are clear indicators to alert your surgeon. Early intervention can often prevent the need for more invasive treatments.
Other Important Warning Signs to Monitor After ACL Surgery
While less common, these issues are also important to watch for:
Nerve or Vascular Issues: Pay attention to numbness tingling after ACL surgery when to worry. Some numbness around the small incision sites is normal and often permanent. However, numbness or tingling that covers a large part of your lower leg or foot, or a cold foot or color changes after knee surgery, could indicate nerve damage or a vascular problem and requires an urgent call to your surgeon.
Wound and Hardware Problems: Besides infection, watch for wound edges pulling apart. Later in recovery, some patients experience screw pain or prominence after ACL surgery. If a screw becomes painful or you can feel it prominently under the skin, it should be evaluated.
Rehabilitation Red Flags: Your progress in physical therapy is a direct window into your knee’s health. Bad signs in acl rehab program include persistent quad weakness (an inability to lift your straight leg) or pain with exercises that should be getting easier.
ACL Surgery Side Effects by Month (Month-by-Month Timeline)
Timeframe | Normal Symptoms | Bad Signs — Seek Help |
Week 1–2 | Peak swelling, pain managed with medication, limited mobility, wound tender | Fever >38.5°C, pus/foul smell from wound, calf pain with swelling (DVT), extreme uncontrolled pain |
Week 3–6 | Reducing swelling, crutch weaning, improved range of motion, some stiffness normal | Cannot straighten knee, knee ‘gives way’, wound reopens, worsening (not improving) pain |
Month 2–3 | Mild aching with exercise, quad weakness improving, slight instability with pivoting | Sharp pain with straight-leg raise, significant giving way episodes, sudden re-swelling |
Month 4 | Occasional deep ache with intensive physio, fatigue in the joint after exercise | 4 months after ACL surgery: persistent instability, inability to jog in straight line, significant pain at rest |
Month 5–6 | Mild stiffness in cold weather, some residual swelling after exercise is normal | Knee pain 6 months after ACL surgery that is worsening or severe at rest — possible graft failure, seek MRI |
Month 6–12+ | Gradual strength return, minor tightness, hardware occasionally felt but not painful | Persistent instability, new giving-way episodes, increasing screw pain — long-term side effects needing evaluation |
ACL Screw Pain Symptoms — Can You Feel the Screws After ACL Surgery?
ACL reconstruction uses small titanium or bioabsorbable screws (interference screws) to anchor the new graft. A common question is: can you feel the screws after ACL surgery? The answer is: in many patients, yes — especially during the first 6–12 months. This is usually not a problem.
Normal Hardware Sensations:
Mild awareness of hardware, especially in cold weather
Slight clicking that resolves without pain
Numbness near the screw insertion point
ACL Screw Pain Symptoms That Need Evaluation:
Screw becoming increasingly prominent — if it pushes against skin and can be felt more over time
Screw pain increasing at 3–6 months — especially with direct pressure or specific movements
Tibial screw pain — pain specifically at the front of the upper shin/tibia where the screw enters
ACL screw pain symptoms alongside new instability — may suggest hardware failure or graft loosening
In a small number of patients, hardware removal surgery may be recommended if screw pain persists beyond 12 months.
Lower Back Pain After ACL Surgery — Why It Happens
Lower back pain after ACL surgery is more common than many patients expect. It typically arises for biomechanical reasons rather than as a direct complication of the surgery itself.
Common Causes:
Altered gait pattern — Walking with a bent knee to avoid pain places stress on the lower spine and hip
Crutch use — Prolonged crutch walking shifts load to the upper back and shoulders
Altered movement patterns — Compensating for ACL knee weakness by overloading the opposite leg and lower back
Reduced core activation — Rest and immobilisation can weaken core stabilisers
When Is Lower Back Pain a Bad Sign?
Lower back pain is rarely a direct surgical complication. However, if it is severe, persists beyond 6 weeks, or is accompanied by numbness/tingling down the leg, evaluation is recommended to rule out nerve involvement.
Fix: Physical therapy for ACL recovery should include core and hip strengthening to address compensatory lower back pain.
It is time to consult Dr. Hesham Al-Khateeb.
Having managed the recovery of thousands of patients, I cannot overstate the importance of listening to your body,” states Dr. Hesham Al Khateeb. “The success of an ACL reconstruction isn’t just about what I do in the operating room; it’s about the partnership between the surgeon, the patient, and the physical therapist during the crucial months that follow. The bad signs after ACL surgery we’ve discussed are your body’s way of telling us that something is off course.”
As the best orthopedic surgeon in Dubai, Dr. Al Khateeb’s approach is built on a foundation of patient education and proactive care. “My advice is simple,” he continues. “Never hesitate to call. A quick phone call can provide peace of mind or, more importantly, allow us to intervene early if there is a real problem. Complications like infection, blood clots, or arthrofibrosis are best managed when caught in their earliest stages. Your vigilance is your best defense.”
If you are in Dubai or the surrounding region and are experiencing any of these ACL surgery complications warning signs, seeking an expert evaluation is the most important step you can take.
Are you concerned about your recovery? Don’t wait. Book a consultation with Dr. Hesham Al Khateeb today to get expert care from one of the best orthopedic surgeons.
FAQs About Bad Signs After aCL Surgery
ACL surgery is not considered high-risk compared to many other surgical procedures. Serious complications — such as pulmonary embolism, severe infection, or complete graft failure — are rare (occurring in under 1–5% of cases each). The procedure is routinely performed arthroscopically as day surgery. That said, like any surgery, it carries risks and your surgeon will discuss your individual risk factors beforehand.
Deaths from ACL surgery are extremely rare. The primary life-threatening risk is a pulmonary embolism (blood clot travelling to the lungs), which occurs in a very small percentage of patients. Modern anticoagulant protocols and early mobilisation have made this risk even lower. General anaesthesia complications are also rare but discussed during pre-surgical consent.
Most patients rate ACL surgery pain as manageable with prescription medication in the first week. The first 3 days are typically the most uncomfortable. By week 2–3, pain should be well controlled with over-the-counter medication. Pain that doesn’t follow this downward trajectory — or that suddenly spikes — is one of the bad signs after ACL surgery and should be reported.
ACL reconstruction has a success rate of approximately 82–90% for return to the same level of sport. Graft re-tear rates are approximately 1 in 4 in young athletes who return to pivoting sports. Success is significantly influenced by adherence to the full physiotherapy rehabilitation programme.
Long-term side effects of ACL surgery can include: mild knee stiffness (especially in cold weather), hardware awareness (feeling the screws), a slightly higher risk of knee osteoarthritis over a decade+ compared to the non-operated knee, occasional aching with high-impact activities, and in some cases graft donor site discomfort (particularly with hamstring or patellar tendon grafts). Most patients with successful rehabilitation report excellent long-term function.
Normal ACL surgery side effects in the first week include: significant swelling, bruising travelling down the leg, stiffness, pain managed with medication, numbness near incision sites, difficulty bearing full weight, nausea from anaesthesia or medication, and fatigue. These are expected and not bad signs. Report: fever above 38.5°C, calf pain/swelling, pus-like discharge, or uncontrollable pain.
Yes, a mild burning or tingling sensation directly around the small incision sites is very common. This is due to the small, superficial skin nerves that were cut during the procedure. This sensation usually fades over time but can sometimes persist. However, if the burning is accompanied by spreading redness, warmth, or severe pain, it could be a sign of infection and should be checked.
This is a crucial distinction. Normal inflammation gradually decreases over time. The pain gets better, the swelling goes down with elevation and ice, and the redness is confined to the immediate incision lines. An infection, on the other hand, gets progressively worse. The pain increases, the swelling is constant or worsening, the redness expands, and the area becomes hot to the touch. The key differentiators for infection are often systemic signs like fever and the presence of pus-like drainage.
It is normal for the operated knee to feel slightly warmer than the non-operated knee for several weeks, and sometimes even a few months, after surgery. This is a sign of the ongoing healing process and increased blood flow to the area. You should worry if the warmth becomes intense heat, is associated with the other signs of infection after ACL surgery (like worsening pain, redness, drainage, or fever), or if it suddenly gets much warmer than it had been.
A sudden pop followed by immediate, significant swelling, especially months into your recovery, is a major red flag. This combination of events closely mimics the mechanism of the original ACL tear and is highly suspicious for a graft re-tear or a new meniscal injury. You should stop your activity immediately, ice and elevate the knee, and contact your surgeon’s office as soon as possible for an evaluation. These are potential retear symptoms after acl surgery.
Swelling becomes an emergency in two main scenarios. First, if it appears suddenly and is extremely large and tense, making it impossible to bend your knee and causing severe pain. This could indicate acute bleeding into the joint. Second, if the swelling is located primarily in your calf (not your knee), is firm, and is accompanied by pain and tenderness. This is a primary sign of a DVT (blood clot), which is a medical emergency. In either of these cases, you should seek immediate medical attention.
Patients with a partial knee replacement (PKR) often retain more of their natural joint function and stability, which may give them a higher chance of being cleared for light jogging compared to those who have had a total knee replacement (TKR). However, even with PKR, high-impact sports should be approached with caution.
About the author
Dr. Hesham Al-Khateeb
FRCS (Trauma & Orthopaedics) – UK
UK Board-Certified
15+ Years Experience
DHA Licensed – Dubai
Knee & Hip Specialist
Medical Disclaimer
This article is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of a qualified health provider with any questions you may have regarding a medical condition.
Medically reviewed by Dr. Hesham Al-Khateeb